Science demands that phenomena be observed with the unemotional accuracy of a weighing machine, while artistic accuracy demands that things be observed by a sentient individual recording the sensations produced in him by the phenomena of life.
Ran across this interesting article, basically saying sight size is easy to teach, and easy to get a good looking result, but the slavish gridding and plotting in the end limit the artist and when the artist is ready to move on beyond student excercises they are at a lack for creating things that aren't in front of them with perfect lighting. Also argues that sight stize was invented in the 20th century, and that Bargue himself didn't use it, even though his litho's are widely used to teach it now.
In teaching, we neglect to
sponsor passion as a discipline.
The only discipline we teach is
that of the deadly diagram
supposedly to be fertilized later
by personal experience.
Later is to late."
(Comparative, by the way, I think is judging distances and proportions by eye (as opposed to sight size having canvas and subject exactly equal, measuring a distance on the subject then shifting over and marking it on the canvas.))